The aim of this proposal is to design and develop new and practical solid state photomultiplier (SSPM) based detector modules, with built-in depth of interaction (DOI) capability, suitable for use in a dedicated breast PET system. Many important detector and system configuration questions will be studied in this project. The work in this proposal will provide the foundation for the ultimate goal of developing a compact clinical PET system for breast imaging that provides images of significantly improved quality at substantially lower cost compared with current commercial PET systems. This will extend the application of PET in the diagnosis and management of patients with breast cancer. The proposed detector module will consists of an array of small discrete lutetium oxyorthosilicate (LSO) scintillator elements, optically connected to two SSPM arrays at both ends. The newly developed SSPM array has high quantum efficiency, uniform and high gain among all elements, and is compact in size with small dead space. This new detector module will provide high spatial resolution, high detection efficiency, excellent count rate performance and coincidence timing resolution, and sufficient depth of interaction resolution to improve the uniformity of spatial resolution across the field of view. We have previously demonstrated the feasibility of the detector design. In this proposal, we will first optimize the detector performance by carefully studying the effects of crystal surface treatment, external reflector and couplings of different components, increase the light collection by reflecting light and detecting them from both ends of the crystal, and pay close attenuation to the trade-off among energy, timing, spatial and DOI resolution. We will optimize the operating conditions of the SSPM array, and use parallel readout electronics to read out each detector channel in order to reduce noise levels. We will construct and evaluate two full detector modules to assess the detector design concepts, as well as the importance of the DOI measurement capability. Finally, the two detectors will be mounted on translational and rotational stages to perform imaging experiments with different geometry (ring and rotating plate), allowing the performance of an entire PET system to be projected. The aim of this project is to investigate a new detector technology to significantly improve the imaging performance of Positron Emission Tomography (PET). The success of this project will lead to the development a dedicated breast-imaging PET system which is expected to provide superior imaging quality at fraction of the cost compared with current commercial general-purpose PET systems. This will have lasting values in its social and economic benefits to the patients and public health with substantially improved clinical diagnosis accuracy at lower cost, and new research potential and impact to the advances of PET technology as well.